Doctors aren't only handing out too many antibiotics, they also are frequently prescribing the wrong ones, researchers and public-health officials say.

Recent studies have shown that doctors are overprescribing broad-spectrum antibiotics, sometimes called the big guns, that kill a wide swath of both good and bad bacteria in the body. Instead, narrow-spectrum antibiotics, like penicillin, amoxicillin and cephalexin, can usually clear...